First time you sold some of your D&D books, for whatever reason.
I've never actually sold any of them, partly because we really used the hell out of most of the ones that would have much resale value. My copies of the rulebooks from the various boxed sets are dog-eared, worn, and have spines of duct tape. I've got battered and beaten modules and Gazetteers and sourcebooks, too. A few might be considered fair to good condition. But that's only part of the reason why I haven't sold any. Mostly, I just don't want to give them up, even the ones I've barely used. There's too much nostalgia, too many fond memories of thumbing through them for hours for inspiration and entertainment. There's also the hope that they may still be useful in the present and future, too. My D&D days are not behind me yet - not by a long shot.